Make basic RadioButton's size easily configurable

As documented in https://forum.qt.io/topic/80402/change-size-of-radiobutton-qt-quick-controls-2, changing the indicator height and width leads to an inappropriately-sized control when the button is checked.

When looking at the `basic/RadioButton.qml` button, https://github.com/qt/qtquickcontrols2/blob/dev/src/quickcontrols2/basic/RadioButton.qml#L69, it seems that the width and height of the pressed button are hardcoded. 

If the child Rectangle width and height were set to a ratio, for instance `width: parent.width*5/7` and `height: parent.height * 5/7`, then this would not modify the default appearance, but it would allow for simple control via `indicator.width` and `indicator.height`.